Geothermal fluid exiting production wells is separated into a steam portion and a brine portion at many prior art geothermal based power plants. The steam portion is expandable in a steam turbine to produce power and electricity, and the heat content of the brine portion may be exploited by being brought in heat exchanger relation with the motive fluid of a binary power cycle, e.g. organic motive fluid, to preheat the motive fluid. To increase productivity of the geothermal resource and maintain environmentally safe conditions, steam condensate, spent brine, and non-condensable gases are generally returned to the resource via a reinjection well.
A significant problem associated with the generation of power by means of geothermal fluid is one of clogging and scaling of heat exchangers, which serve to transfer heat from the brine to the organic fluid, by solids precipitated out of the brine.
In many geothermal resources, the geothermal fluid contains dissolved solids, e.g. silica and calcite, and gases, e.g. carbon dioxide and hydrogen sulfide, in equilibrium at the prevailing subterranean temperature and pressure. However when production wells are drilled in order to exploit these resources, the hydrostatic pressure of the geothermal fluid is reduced at a reduced depth, resulting in the flashing of the steam and the liberation of dissolved gases. The brine consequently becomes more concentrated due to steam flashing and degassing, usually exceeding the saturation levels of scale-forming materials in the brine. It has also been discovered that the rate of precipitation increases as the pH level of the brine increases. The brine generally tends to remain supersaturated such that precipitation of the dissolved solids occurs gradually along the brine flowpath, including precipitation of carbonates at or upstream to the well head and at the inner walls of heat exchanger tubing and slower silicon precipitation on the casing of the injection well. This scaling effect reduces power plant output due to reduced heat flux and fluid flow through the heat exchanger tubing, and due to power plant shutdowns in order to perform maintenance operations.
One prior art method for preventing scaling of the geothermal fluid involves pressurizing the brine above the steam flashing and gas liberation pressure by means of a pump positioned deeply within the production well. This method is suitable for low enthalpy geothermal fluids having small amounts of dissolved solids and gases at moderate temperature equilibrium conditions.
When the brine composition and temperature is not suitable for being pressurized by a pump positioned deeply within the production well, for example when the geothermal resource contains a high enthalpy geothermal fluid, another prior art scaling preventing method is related to chemical based scaling inhibition whereby caustic soda or other pH raising chemicals are added to the geothermal fluid by a process which is not cost effective, to control the corrosion rate of the brine-disposal piping and of the reinjection well casing while avoiding excessive scaling. If silica is present in the geothermal fluid, acid is injected in order to reduce pH and to prevent scaling.
Another prior art scaling preventing method concerns the combining of steam condensate and acidic brine. As the steam condensate is less acidic than the brine, the brine becomes diluted, and the degree of scaling is reduced.
In U.S. Pat. No. 5,497,624, brine exiting the preheater is combined with steam condensate and with compressed non-condensable gases to bring about a reduction in the amount of mineral precipitation in the conduits leading to the reinjection well, as well as in the injection well itself.
In U.S. Pat. No. 5,816,048, steam condensate exiting a vaporizer for vaporizing organic motive fluid is mixed with acidic brine exiting a separator, such that the combined stream is introduced to a preheater for increasing the sensible heat of the motive fluid. In one embodiment, the steam condensate is added to the acidic brine before it is applied to a second vaporizer. The non-condensable gases are then compressed and introduced into cooled and diluted brine, and the resulting effluent is injected into the reinjection well.
Despite being diluted, the brine remains somewhat acidic and mineral precipitation continues to be noticeable, although to a smaller extent. The conduits and heat exchangers in contact with the brine have to made resistant to the corrosive effects of the brine, further adding costs to the power plant.
It is an object of the present invention to provide a method for considerably reducing, or completely preventing, the amount of mineral precipitation on power plant equipment and conduits relative to prior art scaling preventing methods when geothermal brine is used to transfer heat to a motive fluid of a binary power cycle.
It is an additional object of the present invention to maximize utilization of heat contained in the brine for power production even though scaling is prevented.
It is an additional object of the present invention to prevent scaling from geothermal fluid without use of, or with very little use of, chemical inhibitors.
Other objects and advantages of the invention will become apparent as the description proceeds.